A display element utilizing the characteristics of liquid crystal materials such as optical anisotropy and dielectric anisotropy has been extensively used in the applications including watch, calculator or the like. Liquid crystal phase includes a nematic phase, a smectic phase and a cholesteric phase. In practical use, the nematic phase is most conventional. The display modes in this case can include a twisted nematic (TN) mode, a dynamic scattering (DS) mode, a guest/host mode, a DAP (deformation of aligned phases) mode or the like. A large number of the liquid crystalline compounds used for the display modes have been developed, but there is no commercial example for a display element in which a single compound is encapsulated. It is required that liquid crystal materials for display element exhibit a liquid crystal phase over a broad temperature range centering in room temperature, have a good stability towards moisture, light, heat, air, electric fields and electromagnetic radiation under the environment in which a display element is used, and have sufficient characteristics to drive a display element.
The physical values such as optical anisotropy, dielectric anisotropy and electrical conductivity required for liquid crystal materials are dependent on mode of the display and configuration of the element. In particular, the liquid crystal materials for the STN mode which have been recently applied to the liquid crystal display of high quality level should have high elastic constant ratio (k3/k1) and dielectric anisotropy, and low viscosity to obtain good display having high steepness and quick response rate.
However, a single compound satisfying these requirements at the same time is not known yet and the liquid crystal materials used in the current display are the composition comprising a mixture of liquid crystalline compounds having each of the characteristics. If a single compound having different characteristics is mixed each other, frequency and temperature dependences are higher, which makes it difficult to obtain uniform display under each use environment. Thus, a compound having high elastic constant ratio, high dielectric anisotropy, optical anisotropy close to the predetermined constants, broad liquid crystal temperature range, high compatibility with other liquid crystals, good stability and low viscosity not injuring a response rate is an important key to obtain a display of the STN mode having very reduced frequency and temperature dependences and good characteristics.
As the liquid crystalline compounds having high elastic constant ratio, high dielectric anisotropy and relatively low viscosity, alkenyl compounds of the following formula (A) are generally known in Japanese Patent Kokai 59-176221. Those compounds are comparatively narrow in the nematic liquid crystal temperature range and so require a combined use with the compounds having high clearing points to compensate the narrow temperature range, where they are used as an ingredient of the liquid crystal composition. In general, however, the compound having high clearing point exhibits high viscosity. Therefore, an addition of the above-mentioned alkenyl compounds results in an increase in the viscosity of the total composition. Cinnamonitrile derivatives of the following formula (B) wherein R is alkyl of 1-8 carbons are also known as a liquid crystalline compound in Japanese Patent Kokai 55-9012, but they may have poor stability towards light. Further, propiolonitrile derivatives of the following formula (C) wherein R is a straight-chain alkyl of 1-9 carbons or a straight-chain alkoxy of 1-9 carbons are known in Japanese Patent Kokai 58-110527, which have broad liquid crystal range, relatively high dielectric anisotropy and elastic constant ratio, but further improvement may be desired. ##STR2##